1. Field of the Invention
At least one embodiment of the invention generally relates to a heart monitoring and/or therapy device that is configured to detect fusion beats.
2. Description of the Related Art
Typically, fusion beat represents the merging of two different cardiac excitation waves. For patients with implantable cardiac pacemakers or defibrillators, generally, fusion beat is often caused by the merging of intrinsic cardiac depolarization and the excitation wave generated by the pacing device that has generated a ventricular pace (VP).
Generally, a ventricular pace may cause a stimulated ventricular depolarization of the heart muscle cells and thus a contraction of the myocardium if the strength of the ventricular pace is above a stimulation threshold. Typically, such supra-threshold ventricular pace (ventricular stimulation pulse, VP) may capture the ventricular myocardium unless the cells of the ventricular myocardium are refractory. A sub-threshold ventricular pace will typically not cause capture.
In demand pacemakers, generally, ventricular paces (ventricular stimulation pulses) are only generated if the pacemaker does not detect a natural (intrinsic) depolarization and/or contraction of the myocardium within a certain time window. However, there is always a possibility that a ventricular pace is issued while there is also an intrinsic depolarization. The simultaneous occurrence of paced and intrinsic depolarization typically results in fusion beats.
Fusion beat recognition is generally important for implantable cardiac devices for several reasons. First, cardiac stimulation that leads to fusion, typically, may represent a waste of battery power. Second, fusion beat, typically, may disrupt the normal cardiac conduction and lead to inefficient myocardial contraction. Third, typically, for an implantable device that implements automatic pacing capture management based on analysis of evoked response (that is the electrogram resulting from a stimulated (paced) depolarization), fusion beat is undesired because it may distort the electrogram signal.
On the other hand, intentional fusion pacing (as opposed to unintended fusion pacing) has been proposed as an alternative pacing modality for cardiac resynchronization therapy (CRT). In heart failure (HF) patients with left bundle brunch block (LBBB) but with intact right-side AV conduction, generally, timing the left ventricular (LV) pacing to create electrical fusion with spontaneous right ventricular (RV) activation may lead to equivalent or better hemodynamic improvement compared to conventional biventricular (BiV) pacing.
Therefore, fusion beat detection generally poses a particular technical challenge for implantable cardiac devices. The cardiac excitation after a pacing pulse typically may take the form that varies between a completely captured one (no intrinsic component) to a completely intrinsic one (no capture component), and the range may be viewed as a continuous spectrum of fusion with varying degrees. The clinical term pseudo-fusion generally refers to the superimposition of an ineffectual pacing spike on an intrinsic waveform. Hence, pseudo-fusion is generally viewed as a special case of fusion that is close to the intrinsic side of the spectrum.
Knowing the degree of fusion can be valuable for cardiac rhythm management. For example, generally, right ventricular (RV) apical pacing causes non-physiological ventricular activation. Special algorithms have been developed to minimize the RV pacing. However, the RV pacing statistics (e.g. number and frequency of RV pacing) maintained by the pacemaker may be misleading: Patient A who has 20% RV pacing may have lower risk of pacing-induced HF than patient B who has only 10% RV pacing—if the RV paces in patient A are mainly fusion beats dominated by intrinsic activity, whereas the majority of RV paces in patient B are fully captured beats. In another example, to maximize the efficacy of CRT, generally, a common strategy is to maximize the percentage of BiV or LV pacing. However, the BiV or LV pacing percentage generally may not reflect the true effectiveness of CRT, because that metric could be inflated by fusion beats, which may be common during episodes of atrial fibrillation with intrinsic AV conduction.
Generally, several methods have been proposed for detection of fusion beats by extracting the morphological features of the intracardiac electrogram (IEGM) signal (e.g., signal peak, slope, duration, zero-crossings, area, etc.) and comparing them to those obtained from the captured and/or intrinsic IEGM templates.
For example, U.S. Pat. No. 6,950,704 entitled “Use of ER Signal Variability for Fusion Detection and Response in Ventricular and Atrial Autocapture Algorithms”, and U.S. Pat. No. 7,006,869 entitled “Method and Device for Enhanced Capture Tracking By Discrimination of Fusion Beats”, both issued to Bradley, appear to disclose wherein fusion beat detection is performed based on template matching. In particular, specific features (or feature statistics) may be extracted from the post-pacing electrical response, such as the paced depolarization integral, the maximum positive slope, etc. Capture (CAP) template and loss-of-capture (LOC) template may respectively be created by overdrive pacing with supra-threshold amplitude and sub-threshold amplitude, respectively. According to Bradley, a beat may be identified as fusion if the extracted features of the post-pacing electrical response do not correspond to those from either CAP template or LOC template. This approach has several limitations. First, selected features cannot fully characterize the morphology of the IEGM signal. Second, although the CAP template is generally stable, the LOC response may show great variations due to multiple factors, for example, the amplitude-dependent pacing artifact, the timing of intrinsic activity after the LOC pace, etc. Therefore, a stable LOC template may not be available.
U.S. Pat. No. 6,904,321 to Bornzin et al., entitled “System and Method of Identifying Fusion for Dual-Chamber Automatic Capture Stimulation Device”, appears to disclose wherein the far-field signal present in the atrial channel is examined for evidence of a far-field R wave whenever the ventricular channel detects a LOC. According to Bornzin et al., if a far-field R wave is present, then fusion is confirmed. If a far-field R wave is absent, then LOC is confirmed. This method assumes (a) fusion beat is always accompanied by far-field R wave in atrial IEGM, and (b) far-field R wave is absent in case of LOC. However, either assumption may not be true. Depending on the degree of fusion and the sensitivity setting of the atrial channel, a fusion beat may not be accompanied by a far-field component in the atrial channel. Contrarily, after a non-capture ventricular pace, the intrinsic ventricular activity (either depolarization or repolarization) may be detected in the atrial channel.
U.S. Pat. No. 7,765,004 to Stalsbert et al., entitled “Methods and Systems for Managing Fusion and Noise In Cardiac Pacing Response Classification”, appears to disclose a method to classify captured beats, fusion/pseudo-fusion beats, and intrinsic activation was disclosed based on examination of signal peaks in one or more detection windows after the pacing pulse. The method of Stalsbert et al. relies on the assumption that the captured, fusion, and intrinsic beat each has its own characteristic profile of peak amplitude and timing. This assumption is not true, in particular for the fusion and intrinsic beat, which may show different peak amplitude and have varying latency after the pacing pulse. A similar approach was disclosed in U.S. Pat. No. 7,979,113 to Dong et al., entitled “Multi Channel Approach to Capture Verification”, disclosing wherein more than one channel are used for beat classification.
In U.S. Pat. No. 6,928,326 issued to Levine, entitled “Diagnosis of Fusion or Pseudofusion”, fusion or pseudo-fusion beat detection may be performed in two sequential steps. After delivering a stimulus, the device first performs capture detection by examining the evoked response. If the device finds the stimulus did not result in capture, then it further uses morphology discrimination algorithm to compare one or more portions of the sensed IEGM to the corresponding portions of a template, which represents the intrinsic waveform. If an adequate match exists between the compared portions, then the beat is classified as fusion and/or pseudo-fusion. According to this approach, no fusion or pseudo-fusion beat detection is performed if capture is detected after the pace. However, a captured pace can still result in fusion beat. That is, the device can capture the local myocardium and sense the evoked response, but the paced waveform can later fuse with the conducted intrinsic waveform. Therefore, this approach can misclassify a fusion beat to a captured beat.